There are two interrelated, major objectives of this project--to definitively describe the lineage of developmental compartments muscle cells traverse during early avian myogenesis, and the effects motor innervation has on these transitions. Investigation into the mechanism of the innervation effect is planned. Other areas to be explored include the development in very young, pre-myogenic limb buds of the ability to respond to later innervation, and, in older embryos and hatched chickens, the different muscle cell populations contained in fast- and slow-twitch muscles and their relation to early embryonic cell types. The assay of choice is in vitro clonal culture of single cells derived from muscle tissue. This technique is used because the differentiated progeny of single cells derived from tissue of any developmental age are likely to reflect the developmental compartment each progenitor cell was in at the time of its removal from the tissue. Since each clone is operationally restricted to a single cell type, the assay allows changes in particular cell populations from heterogeneous tissue to be monitored in the presence of unrelated cell types. The behavior of particular cell populations in response to various experimental treatments is thus easily assayable. BIBLIOGRAPHIC REFERENCES: Hauschka, S.D., C. Hainey, J.C. Angello, T.L. Linkhart, P.H. Bonner, and N.K. White. Clonal studies of muscle development: Analogies between human and chicken cells in vitro and their possible relevance to muscle diseases. In: Exploratory Concepts in Muscular Dystrophy III. Proceedings of the Muscular Dystrophy Conference, Durango, Colorado, 1976, in press.